Method of making lip-type oil seals having improved sealing edge

ABSTRACT

Lip-type oil seals (10) having helical pumping ribs (40) are manufactured by preparing a molded foreproduct of the seal and by trimming a hat portion (62) to form a sharp sealing edge (32). Each of the pumping ribs (40) is configured to include a first section (50) of a uniform cross-section and a bilge-shaped second section (52). Trimming is made along a trim line (68) that intersects in between the length of the first sections (50) to ensure that a length of the first sections (50) is preserved between the sealing edge (32) and the second sections (52) regardless of the tolerance of positioning of a trimming knife. Oil seals having an improved initial static and dynamic sealing performance is obtained. The second sections (52) achieves a satisfactory pumping action even after a substantial wear has taken place.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to oil seals for establishing a sealbetween relatively movable shaft and housing. More particularly, thepresent invention is concerned with lip-type oil seals havinghydrodynamic pumping means and a method of making thereof.

2. Description of the Prior Art

Lip-type oil seals having hydrodynamic pumping elements are disclosed,for example, in U.S. Pat. No. 3,347,554 to Jagger et al and U.S. Pat.No. 3,534,969 to Weinand. The seal includes a sealing lip defined by apair of frustoconical surfaces. Under static conditions of the seal, thesealing lip resiliently engages with the shaft to form a seal bandwhereby a static fluid seal is achieved.

The frustoconical surface facing the atmospheric region is provided witha plurality of helical pumping ribs or screw threads. Under dynamicconditions, the pumping ribs scrape oil film on the shaft andhydrodynamically feed oil back to the sealed region. In certainconventional designs, the pumping ribs have a uniform triangularcross-section throughout the length thereof.

The problem associated with the conventional pumping ribs having auniform triangular cross-section is that the hydrodynamic pumpingcapacity thereof is prematurely decreased with increasing wear.

U.S. Pat. No. 4,094,519 to Heyn et al discloses an oil seal havingso-called "bilge-shaped" pumping ribs. Each of the pumping ribs isdefined by a pair of curved side faces meeting with each other to definea curved working edge. The pumping ribs have a varying cross-section buthave no height at the end near the static seal band.

The advantage of the "bilge-shaped" pumping ribs over the pumping ribsof the uniform triangular cross-section is that a satisfactory pumpingaction is achieved even after a substantial wear has taken place.

However, the problem which must be overcome in manufacturing the oilseals having the bilge-shaped pumping ribs is that it is extremelydifficult to trim the sealing lip with a complete accuracy. Generally,trimming of the seal lips is carried out by a trimming machine wherein atrimming knife is moved along a trim line by a hydraulic cylinder.Unfortunately, the positioning of the trimming knife could not beperfectly accurate and generally involves a tolerance of about 0.2-0.3mm.

If unfortunately the trim line were offset outwardly of the very end ofthe bilge-shaped pumping ribs, the pumping ribs of a substantial heightwould appear on the sealing edge. This causes lifting of the sealingedge away from the shaft and results in a failure of initial staticsealing.

If conversely the trimming knife were inadvertently offset inwardly ofthe end of the bilge-shaped pumping ribs, the pumping ribs would not bebrought into contact with the shaft for a sufficient extent. The resultof this is that the initial hydrodynamic pumping action is insufficient.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodof making an oil seal having a high degree of static and dynamic sealingperformance regardless of the tolerance of trimming.

Another object of the invention is to provide a method of making an oilseal having an improved initial static sealing performance.

A still another object of the invention is to provide a method of makingan oil seal having an improved initial hydrodynamic pumping capability.

A further object of the invention is to provide a method of making anoil seal which achieves an improved hydrodynamic pumping capacity evenafter a substantial wear has taken place.

These objects are achieved by the method according to the inventionwherein a foreproduct of the oil seal incorporating a rigid annularcasing and a molded elastomeric member is first prepared. Theelastomeric member as-molded includes an annular lip forming portion, ahat portion and a plurality of helical pumping ribs formed on thefrustoconical inner surface of the lip forming portion. Each of thepumping ribs includes a first section of a uniform cross-section havinga controlled height and a bilge-shaped second section.

The foreproduct is then subjected to trimming to remove the hat portionand to form a sharp sealing edge. Trimming is performed in such a manneras to cut in between the length of the first sections as molded.

As the first sections of the pumping ribs as molded have a predeterminedaxial extent, a length of the first sections is always preserved betweenthe sealing edge and the second sections even if trimming was conductedwith a certain tolerance. Since the first sections are of a controlledheight, they are readily flattened upon installation to form acontinuous static seal band to thereby establish a satisfactory initialstatic seal.

The length of the first sections in contact with the shaft but lyingoutside the static seal band achieves the dynamic pumping action toestablish a satisfactory initial dynamic seal.

These features and advantages of the invention, as well as otherfeatures and advantages thereof, will become apparent from the followingdescription.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary cross-sectional view showing the oil seal madeaccording to the method of the invention;

FIG. 2 is an elevational view showing the portion of the oil sealencircled by the circle II in FIG. 1;

FIG. 3 is a cross-sectional view taken along the line 3--3 of FIG. 2;

FIG. 4 is a cross-sectional view taken along the line 4--4 of FIG. 2;

FIG. 5 is a perspective view showing a pumping rib as viewed in thedirection of the arrow 5 of FIG. 2;

FIG. 6 is a schematic cross-sectional view showing a part of theforeproduct of the oil seal prior to trimming, it being understood thatonly one of the pumping ribs is shown to simplify the drawing;

FIG. 7A is a schematic view showing the cross-section and the contactpattern, prior to wear, of the sealing lip and pumping ribs of the oilseal according to the invention;

FIG. 7B is a schematic view similar to FIG. 7A but showing thecross-section and contact pattern after a substantial wear;

FIGS. 8A and 8B are views similar to FIGS. 7A and 7B, respectively, butshowing the conventional oil seal having pumping ribs of a uniformcross-section;

FIG. 9 is a graph showing the variation in the pumping rate of the oilseal according to the invention and of the conventional oil seals;

FIGS. 10 and 12 are perspective views similar to FIG. 5 but showing themodified forms of the pumping ribs; and,

FIG. 11 is a graph showing the variation in the axial length of thecontact pattern formed by the pumping ribs shown in FIGS. 5 and 10.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, the oil seal 10 includes an annular casing 12 made,for example, by stamping of a sheet metal blank. Similar to theconventional design, the casing 12 has a tubular mounting portion 14 anda radial bonding flange 16. Bonded to the casing 12 is an elastomericmember 18 having an axially extending portion 20 adapted to beinterference fit within a housing, not shown, and an annular sealingportion 22 sealingly engageable with a shaft as described later.

The sealing portion 22 includes a primary sealing lip 24 and a dustsealing lip 26. The primary sealing lip 24 is defined by a pair offrustoconical surfaces 28 and 30 meeting with each other to form a sharpsealing edge 32. According to the terminology commonly used in the art,the frustoconical surface 30 facing the sealed region 34 will bereferred to as the "oil side" surface and the opposite surface 28directed to the atmospheric region 36 as the "air side" surface. Aconventional garter spring 38 is mounted within a spring groove formedon the outer periphery of the primary sealing lip 24.

The air side surface 28 is provided with a plurality of helical pumpingelements or ribs 40 which are circumferentially evenly spaced apart fromeach other along the frustoconical air side surface 28. Each of thepumping ribs 40 extends obliquely to the axis 42 of the seal 10 at anequal helix angle.

Referring to FIGS. 2-5, each of the pumping ribs 40 is defined by a pairof opposite side faces 44 and 46 that mate with each other to form aworking or oil scraping edge 48. Each of the pumping ribs 40 isconfigured to include an axially inner first section 50 and an axiallyouter second section 52 which are integral with each other. The term"axially inner" or "axially inwardly" as used herein is intended to meanthe direction toward the oil side of the seal and the term "axiallyouter" or "axially outwardly" the direction toward the air side.

The first section 50 has a uniform triangular cross-section throughoutthe length thereof and has a predetermined controlled height, preferablyof about 0.01-0.1 mm, as measured from the plane of the frustoconicalsurface 28. In contrast, the second section 52 is somewhat bilge-shapedand has a varying cross-sectional dimension. The height of the secondsection 52 progressively increases from the juncture 54 of the first andsecond sections, then culminates at the middle portion and thereaftergradually decreases to become zero at the outermost point 56.

The oil seal 10 may be made by molding of an elastomeric material suchas synthetic rubber and by trimming of the molded elastomeric member.Molding may be carried out by the conventional injection molding processby using an inner core provided with a plurality of helical groovesdesigned and configured to mold the pumping ribs 40. Alternatively,compression molding process or transfer molding process may equally beused. The grooves of the inner core are complementary in shape to thepumping ribs 40 shown in FIGS. 2-5, except that the longitudinal lengthof the portions of the grooves corresponding to the first sections 50 isdimensioned in such a manner that the longitudinal length L of the firstsections 50 as molded is sufficiently longer than the final length ofthe first sections and is preferably equal to about 2 mm.

For molding, the casing 12 is first placed in position in the molds andan uncured elastomeric material is then injected into the mold cavity ina well-known manner. The molded elastomeric material is subjected tocuring whereupon the elastomeric material is bonded to the casing 12.After cooling, the molded product is released from the molds to obtain aforeproduct or semiproduct of the oil seal.

Referring to FIG. 6 wherein the foreproduct prior to trimming is shown,the elastomeric member 58 as molded of the foreproduct includes a lipforming portion 60 and an integral hat portion 62 connected to a sprueportion 64. The inner wall 66 of the lip forming portion 60 is a moldedsurface which is defined by the outer surface of the inner core. Thepumping members 40 as molded lie on the frustoconical molded wall 66throughout the entire length thereof. The first section 50 as molded ofeach of the pumping members 40 extends throughout the longitudinallength L.

Referring further to FIG. 6, the elastomeric member 58 as molded is thencut by a trimming knife of a conventional trimming machine, not shown,along a frustoconical trim plane 68 to thereby form a sharp sealing edge32 defined by the oil side surface 30 and the air side surface 28. Thetrim line 68 is so set that it intersects the length L of the firstsection 50 as molded, preferably at about the middle of the length L. Asthe longitudinal length L of the first section 50 is equal to about 2mm, the trimming knife is well able to cut in between the length L ofthe first section 50 as molded even if the tolerance of positioning ofthe trimming knife is 0.2-0.3 mm. In this way, the first sections 50 ofa controlled height are formed for a certain axial extent on the airside surface 28 between the sealing edge 32 and the bilge-shaped secondsections 52, regardless of any tolerance that may be encountered inpositioning of the knife.

In use, the mounting portion 20 (FIG. 1) of the seal is press fit in awell-known manner within a housing, not shown. The sealing lip 24 of theseal is interference fit over a shaft as schematically shown in FIG. 7Awherein the phantom line 70 indicates the outer periphery of the shaft.Preferably, the inner diameter of the sealing edge 32 and the springforce of the garter spring 38 are selected such that the sealing edge 32is slightly compressed to form a static seal band 72 having an axialwidth of about 0.1 mm, as indicated in the lower part of FIG. 7A.

The portions of the pumping ribs 40 in contact with the shaft present aspinelike contact pattern 74. Under the dynamic conditions of the sealwherein the shaft is rotated in the direction shown in FIG. 7A by thearrow 76, the working edges 48 of respective pumping ribs 40 in contactwith the shaft surface operate to scrape oil film and feed oil past thestatic seal band 72 back to the sealed region 34 by the hydrodynamicpumping action.

During the initial stage of use, the oil seal 10 made according to themethod of the invention provides a satisfactory initial sealingperformance under both static and dynamic operating conditions.

Under the static conditions of the seal, the portion of the firstsections 50 which lies in the static seal band 72 will readily becompressed and flattened against the shaft 70 because the first sections50 are of a controlled limited height and, therefore, the first sections50 would not undesirably lift the sealing edge 32 away from the shaft orexcessively reduce the radial sealing pressure acting on the sealingedge 32. As a consequence, a continuous static seal band 72 will beformed along the entire circumference of the shaft to develop asufficient initial static seal performance.

Under the dynamic operating conditions, those parts of the first andsecond sections 50 and 52 which form the spinelike contact pattern 74will serve as the pumping blade to achieve the initial hydrodynamicpumping action.

The initial static and dynamic seal capacities achieved by the pumpingribs 40 made according to the invention are more than those which areachieved by the conventional pumping ribs consisting only of thebilge-shaped section. With the conventional bilge-shaped design,trimming could not be carried out accurately enough to cut exactly alongthe axially inner end of the bilge-shaped pumping ribs. If the trim linewere unfortunately offset axially outwardly of the axially inner end ofthe bilge-shaped pumping ribs, the initial static sealing action wouldbe insufficient because the sealing edge is lifted. If, to the contrary,the trim line were offset axially inwardly of the inner end of thebilge-shaped pumping ribs, the pumping ribs would not be brought incontact with the shaft to a satisfactory extent. The result of thiswould be that the initial pumping action is insufficient.

After the sealing lip 24 and the pumping ribs 40 have undergone asubstantial wear as shown in the upper part of FIG. 7B, the static sealband 72 will be expanded as shown in the lower part of FIG. 7B. Thefirst sections 50 will no longer exist so that the spinelike contactpattern 74 will be formed solely by the bilge-shaped second sections 52.As the contact surface now lies in a plane that obliquely traverses theregion of the second sections 52 having an increased cross-section, thespinelike contact pattern 74 after wear still has an adequate axialextent so that a satisfactory dynamic pumping action is provided.

For the purposes of comparison, in FIGS. 8A and 8B there are shown thecross-section and the contact pattern, prior to and after wear,respectively, of a conventional oil seal having helical pumping ribs 78of a uniform triangular cross-section. It will be noted from FIG. 8Athat, at the initial stage of use, the conventional oil seal presentsthe static seal band 72 and spinelike contact pattern 76 which arecomparable with those achieved by the oil seal according to theinvention. Accordingly, an initial sealing performance will be providedto a satisfactory degree under both the static and dynamic operatingconditions.

However, after a substantial wear has taken place, the spinelike contactpattern 76 formed by the pumping ribs 78 will be substantiallydiminished in size as shown in FIG. 8B. This is because the cone anglewhich the frustoconical air side surface 28 forms with the axis of theseal is increased as wear proceeds. As a result, the hydrodynamicpumping action performed by the pumping ribs 78 of the uniformtriangular cross-section is considerably reduced upon wear.

The present inventors have carried out a comparative experiment whereinthe oil seal 10 according to the invention, the conventional oil sealhaving pumping ribs of the uniform triangular cross-section as shown inFIGS. 8A and 8B, and the other conventional oil seal having thebilge-shaped pumping ribs were tested for hydrodynamic pumping rate.Each sample was installed between a housing and a shaft and the air sideof the sample was filled with oil under atmospheric pressure. Whilerotating the shaft continuously, the flow rate of oil pumped back to theoil side was measured.

The results are shown in the graph of FIG. 9 wherein the curve Arepresents the variation in the pumping rate as a function of time asmeasured with the oil seal 10 of the invention, the curve B with theconventional oil seal having pumping ribs of the uniform triangularcross-section and the curve C with conventional oil seal havingbilge-shaped pumping ribs.

Referring to the graph of FIG. 9, the oil seal 10 according to theinvention presented a high initial pumping rate as indicated by thecurve A. After wear, the pumping rate was slightly decreased but wasmaintained at a satisfactorily high level. In contrast, the conventionaloil seal having pumping ribs of the uniform triangular cross-sectionrevealed a high pumping rate only in the initial period of use. As wearproceeded, the pumping rate was considerably decreased as shown by thecurve B. The curve C indicates that with the conventional oil sealhaving the bilge-shaped pumping ribs, the initial pumping action is notsufficient. However, the pumping rate was increased with increasing wearof the sealing lip.

In summary, with the oil seal 10 made according to the invention, a highdegree of pumping capacity is achieved upon installation as well asafter wear so that a satisfactory dynamic sealing performance isprovided throughout the life of the seal. In addition, a high degree ofinitial static seal is established as described before with reference toFIG. 7A.

FIG. 10 illustrates the modified form of the pumping ribs. Each of thepumping ribs 40A includes the first section 50 molded and trimmed in amanner similar to the first embodiment. The pumping rib 40A alsoincludes a second section 52A of a triangular cross-section defined by apair of curved side faces 44A and 46A meeting together to form a curvedworking ridge 48A. Throughout substantially the entire length thereof,the second section 52A has a progressively increasing cross-section asit departs from the juncture of the first and second sections. At theaxially outer end of the rib 40A, the side faces 44A and 46A terminateat steep end faces one of which is shown at 80.

In FIG. 11, there are shown the variation in the axial length of thespinelike contact pattern 74 as a function of time. As wear proceeds,the axial length of the contact pattern 74 of the first embodimentbegins to decrease earlier as shown by the curve D because the region ofthe largest cross-section is situated at the middle of the length of thesecond section 52. In contrast, the contact pattern 74 of the modifiedembodiment shown in FIG. 10 outlasts for a longer period of use as shownby the curve E since the height of the second section 52A increases upto near the end thereof. Accordingly, the modified embodiment issuitable in applications wherein a longer service life is desirable.

FIG. 12 illustrates another modified form of the pumping ribs. In thisembodiment also, each of the pumping ribs 40B includes the first section50 of a uniform cross-section. The second section 52B has a constantlydiverging cross-section.

While the present invention has been described herein with reference tothe specific embodiment thereof, it is contemplated that the presentinvention is not limited thereby and various changes and modificationsmay be made therein for those skilled in the art without departing fromthe scope of the invention.

What is claimed is:
 1. A method of making an oil seal having a sealinglip defined by a frustoconical air side surface and a frustoconical oilside surface and having a plurality of helical pumping ribs formed onsaid air side surface, said method comprising:preparing a foreproduct ofsaid oil seal incorporating a rigid annular casing and a moldedelastomeric member bonded to said casing; said elastomeric memberas-molded including an annular lip forming portion and a hat portionjoined with said lip forming portion; the inner periphery of said lipforming portion being defined by a frustoconical molded surface; saidelastomeric member as molded having a plurality of helically extendingpumping ribs raised radially inwardly of the plane of said frustoconicalmolded surface; each of said pumping ribs being defined by a pair ofopposite side faces mating with each other to form a working edge; eachof said pumping ribs having a first section of a predetermined lengthand a second section integral with and located axially outwardly of saidfirst section; said first section as molded having a uniformcross-section of a predetermined height throughout the length thereof;said second section having a cross-section of progressively increasingheight for at least a length thereof situated contiguous to said firstsection; and, cutting said elastomeric member as-molded along afrustoconical trim plane that intersects the length of each said firstsection to thereby form a sharp sealing edge whereby a length of eachsaid first section of a predetermined height is preserved between saidsealing edge and each said second section.
 2. The method according toclaim 1, wherein said first and second sections have a substantiallytriangular cross-section.
 3. The method according to claim 1, whereineach second section is configured to present a largest cross-sectionroughly at the middle thereof.
 4. The method according to claim 1,wherein each second section is configured to present a largestcross-section roughly at an axially outer end thereof.
 5. The methodaccording to claim 1, wherein each second section is configured topresent a progressively increasing cross-section substantiallythroughout the length thereof.
 6. The method of claim 1 wherein saidpredetermined height is between 0.01-0.1 mm.
 7. The method of claim 1wherein said length of said first section is about 2 mm.